A motor generator functioning selectively as an electric motor and a power generator is mounted on a hybrid vehicle or an electric automobile. Such a motor generator includes, for example, a cylindrical rotor fixed to an output shaft supported rotatably around an axis, a motor stator having an inner peripheral surface away from an outer peripheral surface of the rotor with a predetermined space provided therebetween, and a motor housing accommodating the motor stator. A motor stator constituting such a motor generator generally includes a core member and a coil. To insulate the core member and the coil from each other, a coil is wound around a motor bobbin, and the bobbin around which the coil is wound is fitted to the core member. Conventionally, such a motor bobbin is made of a synthetic resin, in general, and polyphenylene sulfide or the like is used as the synthetic resin, for example, in a case of a concentrated winding stator (see, for example, Japanese Patent Application Publication Nos. 2005-102454 and 2002-142399).
However, when the bobbin is made of such a synthetic resin, the lower limit of the thickness of the bobbin is said to be about 0.6 mm, and the recent further increase in efficiency, increase in power output, and reduction in size of motor generators and the like are not necessarily sufficiently coped with. It is said that a bobbin for a motor generator required to have a high efficiency and a large output has to fulfill the following four characteristics simultaneously:
1) being thin (small thickness);
2) preventing ground fault between a coil and a core member (high withstand voltage, partial discharge resistance);
3) withstanding heat generated by the coil (heat resistance); and
4) having mechanical strength.
In particular, the small thickness is considered to be extremely important in the sense that the thinner the bobbin is, the more the coil can be increased, so that the space factor can be increased, and the increase in power output can be achieved.